DE3586566T2 - METHOD FOR REMOVING THE NITROGEN OXYDE AND SULFUR FROM EXHAUST GASES. - Google Patents

Description

Field of the invention

This invention relates to techniques for removing nitrogen and sulfur oxides from exhaust gases. More specifically, this invention relates to techniques for converting nitrogen oxide (NO) from flue gas to nitrogen dioxide (NO2) and for removing the associated sulfur oxides (SOX) and nitrogen oxides (NOX) from flue gas prior to exhausting the flue gas into the atmosphere.

Background of the invention

Recently, increasing attention has been paid to the problems of air pollution. A major source of such pollution is emissions from power plants. For example, in power plant boilers, nitrogen and sulphur oxides are produced by the combustion of fuel in the boilers. The nitrogen oxides can be formed by the pyrolysis of nitrogen-containing compounds in the fuel and can also be formed by reactions of N₂ and O₂ at elevated temperatures (called nitrogen fixation). Normally, nitrogen oxides occur as nitric oxide (NO), but other nitrogen oxides, particularly NO₂, can also occur, usually in small amounts. The oxides of nitrogen are referred to below as NOX. The sulphur oxides occur mainly as SO₂ with small amounts of SO₃. The sulphur oxides are referred to as SOX in the following.

SOX and NOX emissions should desirably be removed from the flue gas before it is released into the atmosphere, since SOX combines with atmospheric water vapor to form sulfuric acids. Similarly, NOX combines with atmospheric water vapor to form nitrogen acids. These acids then fall to the earth as "acid rain" and undesirably increase the acidity of the environment. Nitrogen oxides also contribute to air pollution by participating in the formation of photochemical smog.

One method of achieving relatively low levels of SOX and NOX emissions is to use pure fuels such as light fuel oil or natural gas, which are expensive. Less expensive fuels such as coal produce much higher levels of uncontrolled NOX and SOX pollution. If a cheap method of achieving simultaneous NOX/SOX control were available, then impure fuels such as coal could be used with corresponding economic benefits to operators.

US Patent 4,350,669 describes a method for controlling nitrogen oxides in combustion exhaust gases, which comprises adding oxygen-containing hydrocarbon and thereby oxidizing the nitrogen oxide in the exhaust gas to nitrogen dioxide in the presence of oxygen.

Summary of the invention

The present invention relates to a process for converting NO into NO₂ comprising the steps of contacting a NO-containing gas stream with an injection gas which a peroxyl initiator and sufficient oxygen to achieve the conversion of NO to NO₂.

In another embodiment of the present invention, methods are provided to remove nitrogen oxides and sulfur oxides from a gas stream. Such a method includes the steps of contacting a first gas stream containing nitrogen oxides including NO and NO2 in a molar ratio of NO to NO2 greater than 4 and sulfur oxides in a conversion zone with an injection gas containing oxygen and a vaporized peroxyl initiator. The oxygen and vaporized peroxyl initiator are present in an amount sufficient to convert NO to NO2 in the conversion zone to thereby produce a resulting gas stream exiting the conversion zone comprising NO and NO2 in a molar ratio of less than about 2. In an absorption zone, the resulting gas stream is contacted with a particulate sorbent for nitrogen and sulfur oxides in order to remove these nitrogen and sulfur oxides from the gas stream.

The methods described above can be carried out using an apparatus described below.

An apparatus according to one embodiment has two sections: a conversion section for converting NO to NO₂ and an absorption section for removing SOX and NOX from the gas stream exiting the conversion section. The NO to NO₂ conversion section has a gas line with an inlet and an outlet and a gas contacting section therebetween. Means are provided for introducing a first gas stream containing NO, NO₂ and sulfur oxides into the inlet of the gas line. Means are also provided to introduce an injection gas containing a peroxyl initiator and oxygen into the contacting section of the gas line to contact the gas stream containing NO, NO₂ and sulfur oxide. The peroxyl initiator and oxygen are present in sufficient quantities to convert NO and NO₂ to produce a second gas stream which exits the contacting section. The absorption section has means for receiving the second gas stream as it exits the NO to NO₂ conversion section and means for introducing a substantially dry particulate sorbent into the second gas stream. The sorbent removes sulfur and nitrogen oxides from the gas stream. Finally, means are provided for removing the sorbent which has reacted and any sorbent which has not reacted from the second gas stream to produce a clean exhaust gas stream which is discharged to the atmosphere.

Short description of the characters

These and other features, aspects and advantages of the present invention will be better understood when considering the following detailed description, the appended claims and accompanying drawings. In the drawings:

Figure 1 is a schematic representation of an embodiment of a boiler and associated pollution control equipment suitable for practice in accordance with the principles of the present invention;

Fig. 2 is a schematic sectional view taken along section line 2-2 in Fig. 1;

Fig. 3 is a schematic partial sectional view taken along section line 3-3 in Fig. 1;

Fig. 4 is a cross-sectional view of the preheater shown in Fig. 1;

Fig. 5 is a schematic representation of the apparatus used to carry out Example 1;

Fig. 6 is a schematic representation of the apparatus used to carry out Example 2;

Fig. 7 is a schematic representation of the apparatus used to carry out Example 3;

Figures 8 and 9 each show a graphical representation of the results of Example 3 under the following conditions: SO₂ input concentration 475 ppm; O₂ output 4%; CO₂ input 12%; H₂O input less than 1%; argon carrier - balanced;

Fig. 10 is a graphical representation of the percentage removal of NOX and SOX as a function of the stoichiometric ratio of the sorbent used; and

Fig. 11 is a schematic representation of a NOX sorption system useful in the practice of the present invention for installation below a bag filter housing.

Detailed description

In Fig. 1 there is shown an exemplary embodiment of a boiler flue gas contamination control system suitable for practice in accordance with the principles of the present invention. A boiler 10 fired with either coal or oil has a burner section 12 in which air supplied by a fan 14 is burned, for example in burners 15, to produce a furnace gas at a temperature of about 1204.4ºC.

The furnace gas (flue gas) contains combustion products from the burners, unburned fuel, and air; and also typically contains undesirable amounts of SOX, NOX, and contaminants depending on the composition of the fuel burned. Techniques are provided in accordance with the practice of the principles of the present invention to remove such contaminants containing SOX and NOX prior to discharging the flue gas to the atmosphere. The flue gas flows from the burner section 12 downstream through a hanger section 16 of the boiler to thereby heat fluid flowing through tubes 18. The flue gas, in one embodiment, has a temperature of approximately 648.9°C when it leaves the hanger section and flows through conduit 20, i.e., the rear boiler cavity. From the rear boiler cavity 20, the flue gas passes through a convection section 22 of the boiler, where its temperature drops as it heats fluid flowing through the convection section tubes 23. In one embodiment, the flue gas temperature at the exit of the convection section is about 426.7°C. The flue gas flows from the convection section through an air preheater 24 to preheat the air supplied to the boiler and then into an absorption section of the system (generally designated 25) which in the illustrated embodiment has a bag filter housing 26. In one embodiment, the flue gas has a temperature of about 162.8°C upon entering the absorption section of the system. In the absorption section, any NOX and SOX in the flue gas is removed by means of a particulate sorbent (absorbent) for such NOX. and SOX are removed. The flue gas passes through bag filters 28 (only one shown) in the bag filter housing where the NOX and SOX are removed along with entrained materials including the particulate sorbent. In the embodiment shown, a clean flue gas is discharged to the atmosphere at the bag filter housing via a line 30 which directs the flue gas to the environment.

Typically, NOX as it exits the burner section 12 contains about 95% NO and about 5% NO2. It has been found that when the NO to NO2 ratio in the flue gas is reduced to values below about 2, surprisingly high levels of NOX removed can be observed using a particulate absorbent described below as being suitable for practicing the present invention. Therefore, according to a first technique provided by the present invention, NO in the flue gas is converted to NO2. The conversion is accomplished by contacting the NO-containing flue gas stream with an injection gas containing both a peroxyl radical initiator material (HO2) and sufficient oxygen to convert NO to NO2. Preferably, as described in more detail below, the peroxyl initiator is heated and vaporized before contacting the NO-containing flue gas.

The reaction (conversion of NO to NO2) takes place in the presence of such a peroxyl radical. A typical conversion reaction when using propane as a peroxyl initiator is, for example:

C₃H�8; + O2 T C₃H�7; + HO2 (Peroxyl) (I)

This reaction occurs at about 450ºC. NO is then converted to NO₂ according to the following reaction:

NO + HO2 TNO&sub2; + OH (II)

It has been shown that these reactions occur to a desired degree only when the percentage of oxygen present is greater than about 3%. For example, it is known that a conversion of NO to NO2 of less than 20% is obtained when 3% excess O is contained in the flue gas, while a conversion of at least 90% can be obtained when the O2 excess in the flue gas is increased to 9% or more. Boilers operate most efficiently with 3 to 5% excess O2 in the flue gas, depending on a number of factors including the tendency of smoke development, the efficiency of the burner in accelerating air/fuel mixing, air preheat loss and/or other factors. It is undesirable to operate a boiler with an excess of O2 of 9% (or more), which would be the desired percentage for conversion of O to O2, since the size of the boiler for a given output would have to be much larger to accommodate the increased amount of flue gas flow rate. In addition, the increase in excess oxygen can result in less efficient boiler operation due to the large amount of heat lost at the stack.

As described in more detail below, techniques are provided in accordance with the present invention that enable a desired high NO to NO2 conversion in the convection section of a conventional boiler at flue gas temperatures of about 426.7°C to about 760°C.

This is possible without increasing the size of the boiler and while maintaining the oxygen concentration of the flue gas in the range of 3-5%. Such NO to NO₂ conversion is an important feature of the present invention.

To accomplish the conversion of NO to NO2 described above, in accordance with the practice of the present invention, and referring again to Figure 1, a conversion system or apparatus, generally designated 32, is installed on the vessel 10. In one embodiment, the conversion system 32 includes an air compressor 34, a source of the peroxyl initiator material such as propane (not shown), a premixer/preheater assembly 35, and a gas injection grid 36. Air from the air compressor is mixed with a peroxyl initiator such as propane in the premixer/preheater and the propane is heated. The heated (vaporized) propane is then passed through a line 38 into a manifold header (not shown) and then into a series of manifold tubes 39 which form the gas injection grid 36. In the illustrated embodiment, the tubes 39 are located in the rear cavity 20 of the boiler 10 directly in the flow direction above the boiler convection section 22.

The injection grid 36 preferably extends into the rear cavity or gas contacting section of the boiler transverse to the flue gas stream transverse to the gas flow direction. The arrangement of the tubes 39 can be better understood by reference to Figures 2 and 3 in addition to Figure 1. The plurality of tubes 39 forming the grid 36 are parallel to one another with their longitudinal axes transverse to the flue gas flow direction. Each tube 39 has a plurality of bores 40 along its length which serve as nozzles for the vaporized propane. As best seen in Figure 1, the bores are preferably oriented so that the flow direction of the heated propane is approximately at an angle of 10° to 20° to an imaginary plane which through the series. This allows the heated propane to be introduced into the flue gas stream without impacting the adjacent distribution tubes, thereby reducing soot formation in the tube and promoting cleanliness of the nozzles. Preferably, the heated propane is introduced in the direction of the flue gas flow as shown. In the present embodiment, the tubes are not coated with soot from the oxidation reactions of the propane because the vaporized propane enters the flue gas below the distribution tubes as viewed in the flow direction.

Figure 4 shows a semi-schematic cross-section of an exemplary embodiment of a premixer/preheater 35 useful in the practice of the present invention. The premixer/preheater shown is a modification of the type sold by the John B. Zink Company and designated as Model TH-210.

Propane (or other such peroxyl initiator) is introduced into the premixer/preheater through a main gas port 45. Air, oxygen or recirculated flue gas or mixtures thereof are introduced at inlet 42. The air/propane mixture, which in accordance with the present invention is provided with excess oxygen in the amount stoichiometrically required to burn the propane, is ignited at a burner gas tip 44, such as by means of an auxiliary conductor 46. The combustion gas produced by the combustion of propane exits the end of a jacket 48 and mixes with the propane introduced into the premixer/preheater through ports 50 (in one embodiment of using the premixer/preheater, the propane is heated to a temperature between ambient temperature and about 426.7°C). The heated, vaporized propane and the excess oxygen (the injection gas) flow from the premixer/preheater through the line 38 into the distribution pipes 39 of the grid 36. From the grid 36, the injection gas is introduced into the line of the rear cavity 20 in the flow direction directly above the boiler convection section 22.

The injection gas is introduced in a sufficient amount at a sufficient rate to create a barrier or blanket of this gas extending transversely across substantially the entire cross-section of the rear cavity (conduit) 20 transverse to the direction of flow of the NO-containing exhaust gas stream. The NO-containing gas stream contacts the injection gas (the vaporized peroxyl initiator and the oxygen) as it (the NO-containing gas) flows through the conduit.

As the NO in the exhaust gas contacts the vaporized injection gas mixture, the NO is converted to NO2 in accordance with the above reaction equations I and II.

In one embodiment of the practice of the present invention, propane is used as the peroxyl initiator. The temperature of the heated propane/oxygen mixture (the injection gas) at the time of injection into the flue gas stream is preferably less than about 426.7°C. At temperatures higher than about 426.7°C, peroxyl radicals may form before the injection gas is introduced into the flue gas stream. Since the lifetime of the peroxyl radicals is less than about 40 milliseconds, such radicals formed prior to introduction into the flue gas may be quenched and thus unavailable for the conversion reaction. Therefore, it is not preferable that the propane (injection gas) in the premixer/preheater 35 or the injection grid 36 be heated to a temperature higher than about 426.7°C.

Preferably, the O₂ concentration of the injection gas is from about 5 to about 20 vol%. At less than about 5%, there is insufficient O₂ to cause the desired high conversion of NO to NO₂ if the boiler flue gas also contains low levels of O₂, which are usually below 5%. Alternatively, it is not economical or necessary to provide oxygen at a level higher than about 20%. An important feature of the present invention is that the oxygen concentration provided at the reaction site (the site of conversion of NO to NO₂) by means of the high oxygen content in the injection gas is sufficient to effect such conversion. This supply of sufficient oxygen is achieved without requiring excess oxygen levels of 9% (or more) in the boiler exhaust gas, thus without increasing the size of the boiler.

The NO-containing gas stream preferably has a temperature of about 426.7°C to about 760°C at the time of contact with the injection gas. At a temperature below 426.7°C, the temperature is not sufficiently high to generate the required peroxyl radicals. Thus, little, if any, conversion takes place. On the other hand, if the flue gas has a temperature above about 760°C, various hydrocarbon radicals predominate and this causes NO to be reduced to nitrogen gas rather than being oxidized to NO₂.

Generally, in boiler systems, the NO-containing flue gas stream has a velocity of about 9.14 m/sec to about 21.33 m/sec. The injection gas is preferably injected into the line 20 across the path of the NO-containing gas stream at a velocity of at least about ten times the velocity of the NO-containing flue gas stream. This high velocity for the injection gas is required in part because that a jacket or barrier of such injection gas extends across the entire flue gas flow path. Thus, all of the flue gas must flow through and contact the injection gas as it passes through the boiler convection section.

In an exemplary embodiment, a NO-containing flue gas stream entering the rear cavity 20 has a temperature of 648.9°C and is provided with a volumetric flow rate of 5.569 l/sec at a velocity of 9.14 m/sec. An injection gas having a temperature below 426.7°C has a velocity of 307 l/sec and is introduced through the manifolds 39, which in this embodiment have a total of 168 injection holes, each hole having a diameter of approximately 0.475 cm. The velocity of the injection gas in this embodiment is approximately 182.8 m/sec. The injection gas forms a jacket over the entire cross section of the rear cavity 20 of the vessel. All flue gases flowing from the burner section 12 into the boiler convection section 22 pass through the injection gas jacket. Contact of the NO-containing flue gas with the injection gas results in a conversion of the NO in the flue gas to NO₂.

Although the vaporized peroxyl initiator material has been described above with reference to propane, it should be understood that other such peroxyl initiator materials may also be used. The term "peroxyl initiator" as used herein includes hydrocarbons, i.e., compounds consisting only of carbon and hydrogen, compounds containing carbon, hydrogen and oxygen (oxygen-substituted hydrocarbons), and materials containing only hydrogen and oxygen, such as hydrogen peroxide. Hydrogen gas may also be used.

Examples of "peroxyl initiators" useful in practicing the principles of the present invention include, but are not limited to, propane, benzene, ethane, ethylene, n-butane, n-octane, methane, hydrogen, methanol, isobutane, pentane, acetylene, methyl alcohol, ethyl alcohol, acetone, glacial acetic acid, ethyl ether, propyl alcohol, nitrobenzyl alcohol, methyl ethyl ketone, propylene, toluene, formaldehyde, camphor, ether, and glycol, and mixtures thereof. In addition, hydrogen peroxide and hydrogen gas may be used as previously described.

It is thought that the use of peroxyl initiators containing oxygen, e.g. methanol, hydrogen peroxide etc. or ether, either alone or in combination with hydrocarbons such as propane, will provide an additional oxygen source which should facilitate the conversion of NO to NO₂ at low levels of excess oxygen in the boiler flue gas.

Example I Conversion of NO to NO₂ in convective boiler passages

Experiments were conducted to convert NO to NO2 in convective passages of a boiler.

Figure 5 shows a schematic of a boiler 51 used in the experiments of this example. Boiler 51 is a 98 kJoule/sec (10 hp) fire tube boiler manufactured by McKenna Boiler Works, aof Los Angeles, California. The heating surface area is 5.95 m² (64 sqft), with 862 kPa gauge (125 psig) steam pressure velocity. The burner is gas fired with a spark igniter.

An injection head 52 made of stainless steel projects into the convective passage 54 of the boiler. The head has in its walls 18 radial injection gas holes 56 (only 6 of which are shown). Air to the head was provided by an air compressor (not shown) rated to provide over 7.3 standard liters/second (7 SCFM) at 138 kPa gauge (20 psig). The head injection pressure for these tests was 34.5 kPa gauge (5.0 psig). The gas injection temperature was approximately 121.1ºC (250ºF) at the end of the head as measured by a thermocouple (not shown) on one side of the head. The calculated gas flow rate at 34.5 kPa gauge (5.0 psig) is approximately 2.5 standard liters/sec (5.3 SCFM), and is between 4% and 8% of the total exhaust flow rate depending on fire conditions.

Detailed temperature measurements were taken in the convective passage region near the head. These tests were performed during injection of both air and propane to obtain temperature measurement information that includes the effects of the injection head under fire conditions.

To conduct these experiments, NO gas was injected through an orifice 57 into a burner 58 at the outlet end of a fan 60. The injection rates of NO propane were controlled by Rotar flow meters (not shown).

Flue gas samples were continuously taken from a sample port 61 on a boiler stack 62 at a rate of approximately 70.8 standard liters/hour (2.5 SCFH). The gas samples were passed through a NO-NOX analyzer and an oxygen detector for excess O2 measurement.

The flue gas flow rate was calculated using two methods. The first method is based on the known rate of NO addition and the measured concentration of NOX in the stack gas.

This method assumes flue gas flow rates that are lower than actual due to the NO destruction that occurs in the flame. The second method was based on the measured increase in the excess oxygen content in the flue gas caused by the addition of a known amount of ambient air additive through the injection head. The amount of NO additive was not varied during the tests.

Using the first method, with 29.2 standard liters/hour (1.03 SCFH) addition of NO gas resulting in 276 ppm NOX at 3% O2, dry, and using the combustion factor of (9565 SCF/MMBTU) at 3% O2, dry for natural gas fuel, the flue gas flow rate was calculated to be 29.4 standard liters/sec (62.2 SCFM) (3% O2, dry), which is equivalent to 117,150 joules/sec (400,000 BTU/hr) fire rate.

Using the second method, with 2.57 standard liters/sec (5.45 SCFM) of ambient air (20.9% O2) added and an equivalent increase in the excess O2 content in the flue gas from 3.4 to 4.2%, dry, the flue gas flow rate was calculated to be 32.2 standard liters/sec (68.2 SCFM) (3% O2, dry). The average of the two methods is 30.8 standard liters/sec (65.2 SCFM) at 3% O2, dry.

A specific comparison of the boiler operating characteristics before and after turning on the injection of air and propane is presented in Tables 1 and 2 below. The first test (shown in Table 1) was conducted at the maximum firing rate of 213,800 joules/sec (730,000 BTU/hr). At an O₂ excess of 2.2%, approximately 2,480 ppm propane was injected via head 52 into the flue gas along with 2.93 standard liters/sec (6.2 SCFM) of ambient air. This resulted in a conversion of 55% of NO to NO₂ at a gas temperature of 732.2ºC (1350ºF). The amount of propane used was approximately 6.8% of the total amount of fuel used. The excess O₂ content of the flue gas was increased to 3.7%.

In the second experiment (shown in Table 2), the fire rate was reduced to approximately 60% of the maximum value, resulting in an increase in the excess O2 content, which would normally be expected. With the O2 at 3.4%, approximately 1900 ppm of propane was injected into the flue gas along with approximately 2.60 standard liters/sec (5.5 SCFM) of ambient air. This resulted in a conversion of 71% of the NO to NO2 at a gas temperature of 656.6ºC (1230ºF). The amount of propane used was approximately 5.4% of the total fuel used. The excess O2 content of the flue gas was increased to 4.2%.

These tests demonstrate that high levels of NO to NO2 conversion can be achieved by injecting a premixed gas containing ambient air and propane into a conventional boiler at a location where the flue gas is within a suitable temperature range of 426.7ºC (800ºF) to 760ºC (1400ºF). Conversion percentages between 55% and 71% were obtained over a wide range of firing conditions and the increase in excess O2 in the flue gas caused by the injection head was limited to less than 1.0% above the initial value.

This conversion of NO to NO₂ was achieved without quenching the flue gas temperatures. For example, in the experiments carried out, the conversion took place in a time period of about 40 m/sec and the flue gas temperature drop caused by the cooling effect of the injection gas was only about 10ºC (50ºF) to about 26.7ºC (80ºF). The temperature rise However, the flue gas and injection gas temperature increase caused by the exothermic oxidation of the propane is from about 104.4ºC (220ºF) to about 154.4ºC (310ºF), thus providing a net increase in the flue gas temperature in the region of the injection point of the head and in the mixing region where the conversion of NO to NO₂ takes place. TABLE I Before Propane Injection After Propane Injection Combustion Rate: Flue Gas Flow Rate: Head Depth: Exhaust Temperature: Injection Pressure: Injection Gas Percent: Propane Rate: Propane/Flue Gas: Flue Gas Oxygen: NO Concentration, 3% O₂, dry: Percent NO Conversion: Standard Liters/Second (Wet) Zero (Dry) Basis TABLE II Before Propane Injection After Propane Injection Firing Rate: Flue Gas Flow Rate: Standard Lit/sec Head Depth: Flue Gas Temp.: Injection Pressure: Percent Injection Gas: Propane Rate: Propane/Flue Gas: Flue Gas Oxygen: NO Concentration, 3% O₂, dry: Percent NO Conversion: Standard Lit/sec (wet) Zero (dry) Base

After the flue gas has passed through the conversion zone where NO has been converted to NO2, referring again to Figure 1, the flue gas with the reduced levels of NO and increased levels of NO2 flows from the convection section of the boiler 10 (the NO to NO2 conversion section) through an air preheat section 24 and then into the absorption section 25 of the system. (Since the conversion of NO to NO2 takes place in the area of the boiler between the grid 36 and the top tubes 23 in the convection section 22, this space is hereinafter referred to as the "conversion section").

The absorption section of the system in one embodiment has means for receiving the gas stream as it exits the NO to NO2 conversion section and means for introducing a substantially dry particulate sorbent into the gas stream to absorb the oxides of sulfur and nitrogen from the gas stream to provide a reacted sorbent. Means are further provided for removing the reacted sorbent and any unreacted sorbent from the gas stream to provide a clean exhaust gas stream that can be discharged to the atmosphere.

In the embodiment shown in Figure 1, a particulate sodium-based sorbent such as TronaR or Nahcolite is introduced into the baghouse filter housing 26 at the entrance (TronaR is Na2CO3 NaHCO3 2H2O, where Nahcolite is NaHCO3). The fresh sorbent is stored in one or more hoppers 64 and passed through a rotating check valve 66 located below the hopper 64 into an air stream generated by a blower 68. A portion of the sorbent which has only partially reacted and which originates from the hoppers 69 of the baghouse filter housing is returned to an air stream generated by the blower 68 via the rotating check valves 67 located below the hoppers 69. Depending on the ash content of the primary fuel, up to 85% of the sorbent can be recycled to the baghouse hopper 69 to increase the sorbent utilization and effectiveness of the NOX/SOX sorption reaction. The remainder of the sorbent from the baghouse hopper 69 is output as saved sorbent material (if desired, the transport medium for the sorbent can be steam or flue gas or the like, or mixtures of steam and flue gas with air). The air stream and the entrained sorbent passes a plurality of holes or nozzles (not shown) in a series of tubes 70 extending into the path of the flue gas stream near the entrance to the baghouse filter housing. It is also possible to provide a flue gas sorbent distribution head or conduits 27 with separate discharge openings 29 to promote uniform distribution of the flue gas and sorbent material to the baghouse filter surfaces. The particulate sorbent reacts with NOX and the oxides of sulfur (NOX) in the flue gas to remove SOX and NOX from the flue gas. The average particle size of the sorbent used is preferably less than about 60 µm to improve gas-solid distribution in the gas stream, which in turn promotes more uniform distribution of the solid particles to the baghouse filter surfaces, resulting in more effective absorption of SOX and NOX from the gas. The flue gas and sorbent particles enter the bag filter housing where the gas passes through the filter surfaces 28 and is passed as pure flue gas through the line 30 into the stack and then released to the atmosphere. The nozzles 29 preferably extend from the header 27 and direct the flue gas upward into the bag filter filter surfaces. The sorbent particles are filtered out of the flue gas by the filter surfaces carried in the bags. During a regular bag cleaning cycle, the sorbent particles plus any unburnable ash are removed from the fuel and fall into the discharge hoppers 69 and are either discarded or recycled for further use. The recycling ratio can be adjusted by appropriately connecting rotary shut-off valves and transfer lines (not shown).

The chemical processes involved in the removal of sulphur and nitrogen oxides from a flue gas using a sodium-based sorbent, such as sodium bicarbonate, can be understood using the following equations:

SO2; + 2NaHCO3 TNa₂SO₃ + H₂O + 2CO₂ (III);

1/2NO2 + Na₂SO₃ TNa₂SO₄ + 1/4N2 (IV)

The overall reaction is given by the following equation:

SO2; + 1/2NO2 + 2NaHCO3 TNa₂SO₄ + H₂O + 2CO₂ + 1/4N2 (V)

It is also the case that the following reaction takes place:

NO2 + NaHCO3 NaNO3; + CO2 + 1/2H₂O (VI)

From the above reaction equations, it can be seen that SO2 must be present in the flue gas for NO2 to be removed by NaHcO3. Nitric oxide (NO) is also absorbed by the NaHCO3 material or by trona material in the presence of SO2, forming NaNO2 as a byproduct in a reaction similar to reaction VI. The NaNO2 byproduct can be oxidized to NaNO3 in contact with the flue gas.

Another reaction that has been observed is the conversion of some nitric oxide (NO) to nitrogen dioxide (NO2) during absorption of NOX and SOX by the dry sodium-based sorbent particles. The chemistry of this reaction is not fully understood, but as described in Example 7, is useful in the practice of the present invention in reducing the total amount of nitrogen dioxide (NO2) released at the SOX/NOX absorption section. To verify that nitrogen dioxide (NO2) is absorbed, experiments were conducted wherein NOX which did not contain nitrogen oxide (NO), as described in Examples 2 and 3.

The conversion of some NO to NO2 during the simultaneous absorption of SOX and NOX by the sodium-based sorbent brings to light the need to remove nitrogen dioxide (NO2) below the sodium-based absorption section. In one embodiment of the present invention, a metal oxide absorption section is used for the removal of NOX in general and NO2 in particular. The metal oxide can be readily regenerated by heating to a temperature above 371.1°C (700°F) to produce an exhaust gas stream containing nitrogen oxide (NO). This exhaust gas stream can be returned to the main burners in the boiler where most of the special nitrogen oxide (NO) is destroyed in the flame zone.

Although the present invention has been described above with reference to the sorbents trona and nahcolite, sorbents other than sodium-based sorbents may also be used, such as calcium-based sorbents such as Ca(OH)2. Additionally, slaked lime prepared with a sodium-based liquid to obtain a dried Ca(OH)2 powder having a sodium-enriched particle surface may be used. In an exemplary embodiment, the sorbent particles contain a mixture of about 15 wt% NaHCO3 and 85 wt% Ca(OH)2. (The sorbent preferably contains at least about 5% NaHCO3, Na2CO3, or mixtures thereof). Alternatively, if desired, instead of introducing the absorbent particles into the exhaust gas stream at the inlet of the bag filter housing, the absorbent may be introduced into the gas stream into a spray dryer, with the outlet of the spray dryer being discharged into the bag filter housing.

In yet another embodiment for practicing the present invention, the absorption zone has two sections. In a first section of the absorption zone, the exhaust gas stream entering the NO to NO2 conversion zone (the second gas stream) is contacted with sorbent particles for oxides of nitrogen and sulfur to thereby remove the oxides of nitrogen and sulfur from the gas stream, which forms a third gas stream. The third gas stream is then passed to a second section of the absorption zone in which the third gas stream and the sorbent particles contact a liquid sorbent for NO2 and sulfur oxides. The liquid sorbent removes NO2 and sulfur oxides not removed by the particulate sorbent and also removes the sorbent particles.

The molar ratio of SO2/NOX has an effect on the amount of SO2/NOX removed from the flue gas by the particulate sorbent used in the practice of the present invention. For example, when NaHCO3 is used, it is preferable that the ratio of SO2/NOX be greater than about 3, and especially preferable that the ratio be greater than about 5. When a high sulfur fuel is burned, the ratio of SO2 can be as high as 30:1, and when a low sulfur fuel is burned, the ratio of SO2/NOX can be as low as 1:1. Thus, by selecting the fuel to be burned, the ratio can be kept within the preferred range in which the dry sorbent is used.

In another embodiment of the present invention, flue gas with reduced content of NO and increased NO₂ content from the conversion section of the boiler to a conventional wet scrubber. In this embodiment, the wet scrubber has a liquid sorbent or sorbents for the removal of SO₂, for example a caustic such as Ca(OH)₂ or CaCO₃, with the addition of NaOH or Na₂CO₃ for enhanced NO₂ removal. Preferably, the caustic contains at least 5 wt% sodium components. SO₂ and NO₂ are removed via the scrubber and the clean flue gas is released to the atmosphere. The removal of NOX in this embodiment can be increased by using EDTA or other known additives such as ferrous sulfate or iron chelate in the scrubbing liquid.

EXAMPLE 2

Referring to Figure 6, a series of tests were conducted with a scale referenced to a reference point to determine the effectiveness of contacting the gas streams with the powdered trona and nahcolite to remove SOX and/or NOX therefrom.

A heated stainless steel reactor tube 70 was closed at its top and bottom ends with fiberglass plugs 72 and 74, respectively. Tube caps 76 and 78 were screwed into the top and bottom ends of the reactor to hold the fiberglass plugs in place. Separately compressed gas cylinders 80 and 82, each containing NO2 (plus dry air), are connected to the top of the reactor tube 70 via an electrically heated sample line 84. The sample line 84 enters the top of the reactor 70 through a hole in the tube cap 76. A control port 86 is provided to inject dry, particulate trona and/or nahcolite into the reactor. A line 88 is connected between the bottom (outlet) of the reactor and a NOX/SOX gas analyzer 89. 88. The composition of the nahcolite used in these experiments was at least about 93% NaHCO₃, between about 1 and 3% Na₂CO₃, about 0.5% NaCl with equilibrium moisture. The composition of the trona used was about 33-37% Na₂CO₃, 22-27% NaHCO₃, 4-8% NaCl, 5-7% Na₂SO₄, 6-10% water insolubles and 12-21% total H₂O.

In general, the experiments were carried out by heating the reactor and sample lines to a desired temperature. After the NOX/SOX gas analyzer was calibrated, valves 90 and 91 at the outlets of cylinders 80 and 82 were then opened to produce a desired flow rate of NO₂ and SO₂ through the apparatus. Cylinders 80 and 82 contained NO₂ and SO₂ in trace amounts mixed with dry air as a carrier gas. The parts per million (ppm) of SO₂ and NO₂ entering the inlet of the reactor vessel were recorded. A dry sorbent was then injected into the reactor via injection port 86. The ppm of SO₂ and NO₂ exiting the reactor were measured and the percentages of NO₂ and SO₂ removed were recorded.

The results of these tests are presented in the following tables. TABLE III Injection Test with TRONA at 87.8ºC (190ºF) Reactor Temperature Time (min) NO₂(IN) (ppm) SO₂(IN) (ppm) NO₂ Removal (%) SO₂ Removal (%) TABLE IV Nahcolite Injection Test at 87.8ºC (190ºF) Reactor Temperature Time (min) NO₂ (IN) (ppm) SO₂ (IN) (ppm) NO₂ Removal (%) SO₂ Removal (%) Table V Injection Test with NAHCOLIT at 204.4ºC (400ºF) Reactor Temperature Time (min) SO₂ (ON) (ppm) SO₂ (OFF) (ppm) NO₂ (ON) (ppm) NO₂ (OFF) (ppm) NO₂ Removal (%) SO₂ Removal (%) Table V (continued) In addition, NAHCOLIT was injected 10 minutes after the start of the experiment Time (min) SO₂ (ON) (ppm) SO₂ (OFF) (ppm) NO₂ (ON) (ppm) NO₂ (OFF) (ppm) NO₂ Removal (%) SO₂ Removal (%)

EXAMPLE 3

Referring to Figure 7, a second series of tests was carried out with a scale relative to a reference point similar to the tests according to Example I.

The test apparatus 92 used in the second series of tests consisted of compressed gas cylinders 93, 94, 96, 98, 100 and 102 containing argon, O2, CO2, SO2, NO2, and argon, respectively. Argon (cylinder 93), O2 and CO2 were initially metered by means of calibrated rotary flow meters 104, 106 and 108, respectively, into a common distribution line 110 via a water column 111 and into line 112. Next, the test gases SO2 and NO2 were metered into the common distribution line 110 via a water column 111 and into line 112. in concentrations (in the argon carrier gas) of 15,000 ppm and 5,000 ppm respectively into line 112 via rotary flow meters 113 and 114.

The mixture of test gases was then passed (in a tube 115) through an oil bath 116 which was maintained at a temperature which could be adjusted between 148.9ºC (300ºF) and 204.4ºC (400ºF). At the outlet of the oil bath, a tee 118 was provided in the sample line 120 to allow sorbent from a fluidized sorbent bed feeder 121 to pass into the sample line above a filter housing 122. The filter housing 122 contained a 15.24 cm (6 inch) diameter filter 124 which was precoated with diatomaceous earth or highly pulverized calcium sulfate powder so that the pressure drop across the filter could be maintained at approximately 747 Pa (3 inches of water) during the tests. The filter housing 122 was insulated and heated to allow adjustment of the gas temperature within the filter. During the duration of the tests, NaHCO₃ powder was deposited on the filter, whereby at the end of the run a cake of sorbent which had partially reacted remained. Each run lasted approximately 15 to 30 minutes. The temperature of the input gases introduced into the filter housing was adjusted by changing the input power applied to the heater tape (not shown), which was located between the stainless steel jacket of the filter housing and an outer insulating layer (not shown) surrounding the filter housing. The temperature of the filter housing was measured by a thermocouple (not shown) placed in the input gas stream approximately 1 inch above the filter. The results of the experiments at different molar ratios of SO2/NO2, 475 ppm SO2, 4% excess O2, 12% CO2, less than 1% H2O, argon balance are shown in Figures 8, 9 and 10. Figure 8 shows the total NOX removal with dry test gases, which is somewhere between about 50% and 70% when the gas temperatures in the filter housing ranged from about 182.2ºC (360ºF) to 404.4ºC (400ºF).

Figure 9 shows that at temperatures in the temperature range of about 171.1ºC (340ºF) to 204.4ºC (400ºF), between about 60% and 90% of the input NO₂ is eliminated in the process described above.

The desired temperature of the gas stream when contacted by the particulate sorbent to remove NOX and SOX is between about 93.3ºC (200ºF) and 232.2ºC (450ºF). The temperature is preferably between about 148.9ºC (300ºF) and 204.4ºC (400ºF) when Nahcolite is used as the sorbent.

EXAMPLE 4 THE EFFECTS OF ADDITION OF WATER VAPOR AT OUTLET CONCENTRATIONS OF SO₂ AND NOX

The effects of adding water vapor at low concentrations on the initial concentrations of SO₂ and NOx can be seen in Table VI below. This experiment was run at a temperature of 113.9ºC (237ºF), a pressure drop of 149 Pa (0.6 inches H₂O) with a sorbent containing 50% NaHCO₃ in a diatomaceous earth filter aid; an inlet concentration of SO₂ of 471 ppm and an inlet concentration of NO₂ of 173 ppm. TABLE VI EFFECT OF ADDITION OF WATER VAPOR ON REMOVAL OF NO₂ AND SO₂ Gas water content (ppm) Initial SO₂ concentration (ppm) SO₂ removal (%) Initial NOX concentration (ppm) NOX removal (%)

The results of this experiment show that a small amount of water vapor improves both the removal of SO2 and NOX.

The amount of NaHCO₃ sorbent used may also be important. For example, it can be seen from equation (V) that it takes two moles of NaHCO₃ to remove each mole of SO₂ and each half mole of NO₂. The stoichiometric ratio of sorbent to SOX is preferably greater than 1 and more preferably greater than 4. As can be seen from the accompanying Figure 10, the percentage of removal of both SOX and NOX using NaHCO₃ as the sorbent with an increase in the stoichiometric proportion of sorbent. Since it can be expensive to operate a baghouse filter with a fresh sorbent in the stoichiometric ratio of 4, it can be anticipated that similar results can be achieved when recycled baghouse filter sorbent and fresh sorbent in the stoichiometric ratio of less than 2 are used. Furthermore, it can be seen that for a given stoichiometric ratio, the percent of NOX removed is greater for the SOX/NOX molar ratio of 6 in the gas than when the molar ratio is 4.

EXAMPLE 5 EFFECT OF ELIMINATION OF THE PRESENCE OF NO2 ON ABSORPTION OF NOX USING NAHCOLITE AS SORIENT

The effects of SO2 removal on the ability to absorb NOX using Nahcolite as a sorbent can be seen in Table VII below. These tests were run at temperatures of 152.2ºC (306ºF) and 216.7ºC (422ºF), using NaHCO3 as a sorbent in a ratio of approximately 1.0 to a filter cloth precoated with highly pulverized CaSO4.

The gas composition was as follows:

2.9% to 3.2% O₂; 14.5% to 14.9% CO₂; 11.0%-11.5% H₂O; and a balance of nitrogen. The gas flow rate was approximately 0.91 to 1.04 m/min (3.0-3.4 ft/min) through the filter cloth. TABLE VII EFFECT OF REMOVAL OF PRESENT SO2 ON ABSORPTION OF NOX USING NAHCOLITE AS SORPTION Bag Filter Temperature Inlet Conc. NOX (ppm) Inlet Ratio SO₂/NOX Outlet Conc. NOX (ppm) Inlet Ratio SO₂/NOX

In these experiments, the NOX input consisted of at least 75% NO₂. As can be seen from Table VII, the removal of NOX was negligible when the input ratio SO₂/NOX was reduced to 0. This indicates that the presence of SO₂ is required for the absorption of NOX by a sodium-based caustic such as Nahcolite.

Although a large percentage of NOX is removed from the gas stream by techniques consistent with the above-described practice of the present invention by contacting the NOX-containing gas stream with dry, sodium-based sorbent, some NO2 may remain. A further technique is therefore proposed in accordance with the present invention to remove NO2 which may remain in the gas stream after treatment with the dry, particulate sorbent.

It is known that NO₂ is absorbed by oxides of the following metals, or alloys of the following metals, which are metals: aluminum, zirconium, nickel, iron, copper, magnesium, titanium and the like. The metal oxide can be provided on a suitable supporting substrate, preferably to provide a specific metal oxide surface area greater than 10 m²/g total sorbent. NO₂ is primarily absorbed when a surface nitrate has a double bond nitrate attachment to the surface. NO is also absorbed, but not as readily as NO₂.

Figure 11 (in addition to Figure 1) shows in schematic perspective a NOx sorption system 130 which may be installed in accordance with the practice of the present invention, for example, in the duct 30 of the baghouse 26. The sorbent-free exhaust gas exiting the baghouse in the duct 30 (see Figure 1) flows through a bed 132 of metal oxide gravel or pellets having a high specific surface area for the metal oxide 133. In an exemplary embodiment, for example, a exhaust gas velocity of 12.19 m/sec (40 ft/sec) across a 1.22 m (4 ft) thick bed produces a residence time of 0.1 sec and results in a gas side pressure drop of less than about 249 Pa (1.0 in H₂O).

During the NOX absorption process, the pellets 133 in the bed 132 slowly run downwards and are possibly discharged from the bed into a hopper 134, which is arranged, for example, under the exhaust chimney 30.

The pellets which have not absorbed NOX and have been discharged into the hopper 134 (the spent pellets) are regenerated by driving the NOX off the pellets in the form of NO. This is done, for example, in a fluidized bed 138. The pellets pass from the hopper 134 through a rotary check valve 136 into a fluidized bed. As the pellets are heated in the fluidized bed, NO is driven off the pellets. The NO-containing exhaust gas of the fluidized bed is discharged at a high temperature, for example about 398.9ºC (750ºF), is recycled via line 139 back to the burners 12 (shown in Figure 1) of the boiler 10. Since all of the NO produced in the boiler is in thermodynamic equilibrium, most of the additional NO introduced by the metal oxide regeneration system is destroyed in the main burner flame. The waste heat from the hot regenerated pellets can be recovered, if desired, by countercurrent heat exchange with the ambient air supplied to the fluidized bed burner system. The cooled regenerated metal oxide pellets can be pneumatically transported to the top of the pellet bed duct train, for example by means of a transport system 140 having a blower 142, a cyclone 144 and a transport line 146 between the blower and the cyclone. The regenerated metal oxide pellets pass from the fluidized bed via a rotary valve 148 into line 146 and are transported to the cyclone 144 at the top of the bed 132. The pellets pass from the cyclone back into the bed via a rotary check valve 150 at the top of the bed. Abrasives can be recovered from the top of the cyclone via line 152 which is connected to a filter 153 via suitable piping (not shown).

EXAMPLE 6 FINAL REMOVAL OF NOX FROM THE EXHAUST GAS BELOW A BAG FILTER HOUSING

Referring to Figure 5, the same vessel used for Example I is also used for the experiments of this example. To contact NOX with a metal oxide for removing this NOX, a tube 160 with 6.1 m (20 ft) length and 0.635 cm (0.25 in) outside diameter was used. made of oxidized aluminum is connected to the exhaust chimney 62 at connection 61.

Approximately 70.8 standard liters/hour (2.5 SCFH) of sample gas was drawn through the tube to contact its Al₂O₃ interior surface. The calculated interior volume of the tube was 108 cm3 (6.6 in3), giving a residence time for the exhaust gas of approximately 90 m/sec. The interior surface area of the tube was 903 cm2 (140 in2).

Short-term sieving tests were conducted to determine the effects of Al₂O₃ on the removal of NO and NO₂ and also to verify that the Al₂O₃ could be regenerated upon heating by driving off NOX in the form of nitrogen oxide gas (NO).

To perform the experiments, NO gas was injected into the burner 58 at the outlet of the blower 60. The NO injection rate was controlled by a rotary flow meter (not shown).

Flue gas samples were continuously withdrawn from the boiler stack 62 through the Al2O3 tube 160 at a rate of about 70.8 standard liters/hour (2.5 SCFH) and passed through two banks of impingement elements in an ice bath to remove excess moisture (not shown). The gas sample was then passed through a NO-NOx analyzer and a portable oxygen detector for measuring excess oxygen. During the tests at an average temperature of about 93.3°C (200°F), the absorption of NOx was very high with a total residence time of about 0.1 sec through the alumina tube. The data shown in Table VIII were obtained during the absorption process at a 4.2% excess oxygen in the gas sample: TABLE VIII Absorption Mode (3% O₂, dry Initial NO: Final NO: NO removal: Initial NOX: Final NOX: NOX removal: continuously decreasing

The reaction of NOX on the Al₂O₃ tubes appeared to produce oxygen off-gas. It is assumed that the Al₂O₃ tube was thoroughly oxidized and the absorption of the NO₂ at the surface layer produced half a mole of O₂ for each mole of NO₂ absorbed. The reaction of Al₂O₃ with NO₂ is shown in the following reaction equation:

Al₂O₃ + NO2; T Al₂O₃NO + 1/20₂

The alumina was regenerated by passing a propane gas flame along the tube (for approximately 5 minutes). During the regeneration stage, oxygen was absorbed and the NO-NOX levels were increased above the input levels. The results of the regeneration tests are shown in Table IX: TABLE IX REGENERATION OF AL₂O₃ TUBE (3% O₂, dry) Initial NO: Final NO: Initial NOX: Final NOX: and increasing

EXAMPLE 7 CONVERSION OF NO TO NO₂ FOLLOWED BY ABSORPTION OF SOX AND NOX IN A BAG FILTER HOUSING

The same vessel used for Example I was used in the experiments of this example, except that a pulse jet baghouse manufactured by EVO Corporation, Model No. NF-9, was placed below the vessel. The baghouse contained 8 bag filters with a total filter area of 40 square feet. The filter material was matted Nomex cloth. The baghouse was fed with a variable speed ID blower to overcome the pressure drop through the bag filters and to balance the draft requirements of the vessel. In this example, the pressure drop across the bag filters was approximately 3 in-H2O and the air/cloth ratio was approximately 15.75 absolute liters/s/m2 (3.1 ACFM/ft).

As shown in Table 10, without the conversion of NO to NO₂ upstream, approximately 24% NOX removal was obtained at a high sorbent stoichiometric ratio between 3 and 4, which also resulted in a high SO₂ removal of 96%. However, the amount of NO₂ increased by 47 ppm as a result of the simultaneous removal of SO₂ and NOX. Note that the initial amounts of NO₂ at the baghouse inlet were a The trick of the process of introducing concentrated nitrogen oxide (NO) gas directly into the boiler system is that much lower levels of NO₂ at the baghouse entrance would normally be expected in a conventional boiler system.

TABLE X COMPARISON OF SO2 AND NOX REMOVAL WITH AND WITHOUT UPPER CONVERSION OF NO TO NO2

The tests were carried out with dry trona powder Without any upstream conversion of NO to NO₂

Bag filter temperature: 310ºF

Initial ratio NO/NO2: 2.7

Initial ratio SO₂/NOX 4.8 Flue gas concentrations (ppmv, corrected to 3% O₂, dry) Bag filter inlet Injection head OFF SO₂ removal: 97% NOX removal: 24%

The tests were carried out with dry trona powder with upstream conversion of NO to NO₂

Bag filter temperature: 160ºC (320ºF)

Initial ratio NO/NO2: 3.4

Initial ratio SO₂/NOX: 4.2 Flue gas concentrations (ppmv, corrected to 3%, O₂ dry) Bag filter inlet Injection head OFF Bag filter outlet Injection head ON SO₂ removal: 94% NOX removal: 46%

The practice of the present invention results in surprisingly high levels of NOX removal in conjunction with SO2 removal. As shown in Table X, upstream of the conversion of NO to NO2, a removal of NOX of about 46% is obtained at a lower stoichiometric ratio of about 2, which also results in a lower SO2 removal of 94%. This improvement in NOX removal also occurs at a lower value of the initial SO2/NOX ratio, further demonstrating the usefulness of the present invention. Furthermore, an increase in NO2 by 36 ppm (compared to the previous result of 47 ppm) indicates that the amount of NO2 resulting from the simultaneous removal of SO2 and NOX was reduced. As described above, the present invention provides a method for removing this NO₂ byproduct using a metal oxide absorption section located downstream.

The foregoing descriptions of exemplary embodiments for removing NOX and SOX from the flue gas streams are for illustrative purposes only. The present invention is not intended to be limited to the particular embodiments described above, as variations are possible, as will be apparent to those skilled in the art.

For example, it would be possible to accomplish NOX reduction in the absence of SO2 by converting NO to NO2 in the convection section of the boiler by injecting a peroxyl initiator as described above and then to accomplish NOX absorption using a regenerable metal oxide. In this embodiment, for example, regenerable metal oxides as described above can be introduced in powder form into the absorption zone to contact a flue gas stream containing nitrogen oxides such as NO2. The metal oxide removes the nitrogen oxides from the gas stream and is separated from the gas stream in, for example, the baghouse. The metal oxide can be regenerated for reuse by heating to a temperature of at least about 371.1°C (700°F). Such heating produces an exhaust gas containing NO which can be recycled to the burner section of the boiler. The scope of the present invention is defined by the following claims.

Claims (25)

1. A process for converting NO from exhaust gases to NO₂, consisting of contacting an NO-containing exhaust gas stream in a gas contact section downstream of the combustion chamber with an injection fluid, characterized in that the injection fluid contains premixed amounts of a peroxyl initiator and sufficient oxygen to provide the conversion of NO to NO₂, the premixing occurring outside the gas contact section. 2. Method according to claim 1, characterized in that the injection fluid contains at least 5 vol.% oxygen based on the total volume of the injection fluid. 3. Method according to claim 2, characterized in that the injection fluid contains approximately 5-20 vol.% oxygen based on the total volume of the injection fluid. 4. Method according to one of the preceding claims, characterized in that the oxygen content of the exhaust gas stream after mixing with the injection fluid is not increased by more than 1% of its initial value. 5. Method according to one of the preceding claims, characterized by: (a) passing an exhaust gas stream containing nitrogen oxides, including NO, through a conduit; b) introducing an injection fluid in sufficient quantity at a sufficient number of injection points and at a sufficient speed into the line to cause such injection fluid to extend substantially over the entire line cross-section in a plane substantially transverse to the flow direction of the NO-containing gas stream, the NO-containing gas stream thereby contacting the injection fluid as the NO-containing gas stream flows through the line. 6. A method according to claim 5, characterized in that the injection fluid is introduced in a conventional boiler at one or more locations in the heat-conducting section or the rear cavity areas to contact the NO-containing gas stream. 7. A method according to any one of claims 5 and 6, characterized in that the injection fluid is introduced into the line through a plurality of nozzles, the exit velocity of the injection fluid from each of these nozzles being at least ten times the velocity of the NO-containing gas stream in the vicinity of the nozzles. 8. A method according to any preceding claim, characterized in that the injection fluid is preheated to an injection temperature of less than about 427ºC. 9. A method according to any one of the preceding claims, characterized in that the exhaust gas stream has a temperature higher than about 427ºC at the time of contact with the injection fluid. 10. A method according to claim 9, characterized in that the exhaust gas stream at the time of contact with the injection fluid has a temperature of approximately 427ºC to 760ºC. 11. A method according to any one of the preceding claims, characterized in that the exhaust gas stream entering the gas contact section contains NO and NO₂ in a molar ratio of approximately 4 at most and the gas stream leaving the gas contact section contains NO and NO₂ in a molar ratio of approximately 2 at most. 12. Process according to one of the preceding claims, characterized in that the peroxyl initiator is selected from the group consisting of hydrocarbons, oxygen, substituted hydrocarbons, hydrogen and hydrogen peroxide and mixtures thereof. 13. Process according to claim 12, characterized in that the peroxyl initiator is methanol. 14. Process according to claim 12, characterized in that the peroxyl initiator is selected from the group of hydrocarbon gases consisting of methane, ethane, hydrocarbon gases or mixtures of the same. 15. The method of claim 14, characterized in that the combustible peroxyl initiator material is combusted with additional air or oxygen to produce a preheated, premixed injection fluid. 16. A process for removing nitrogen and sulphur oxides from an exhaust gas stream, comprising the steps of a) converting NO in the exhaust gas into NO2 in a conversion zone; b) passing the resulting gas stream through an absorption zone for the removal of SO₂, in which NO₂ is also removed, characterized in that step a) is carried out by a method according to one of the preceding claims. 17. Process according to claim 16, characterized in that the absorption zone contains a wet cleaner. 18. Process according to claim 17, characterized in that lime or limestone reactants are used in the wet cleaner for the removal of SO₂ and NO₂. 19. Process according to claim 17, characterized in that sodium-based alkali reactants, such as NaH or Na₂CO₃, are used in the wet cleaner to remove SO₂ and NO₂. 20. Method according to one of claims 17 to 19, characterized in that additives selected from the group consisting of EDTA iron (II) sulfate and iron chelate and mixtures thereof are used in the cleaning liquid to improve the removal of NOX by means of the wet cleaner. 21. Process according to claim 16, characterized in that the absorption zone contains a particulate sorbent. 22. The method of claim 21, characterized in that the method of contacting the resulting gas stream with particulate sorbent comprises the step of injecting particulate sorbent into the exhaust gas above a bag filter housing by means of either a spray dryer or a pneumatic feeder and recycling a portion of the sorbent collected in the bag filter housing. 23. A process according to any one of claims 21 and 22, characterized in that the particulate sorbent is selected from the group consisting of trona, nahcolite, lime, hydrated lime, sodium-enriched lime, fly ash, wet slurries thereof suitable for use in spray dryers, or combinations thereof with waters of hydroaccumulation. 24. Process according to one of claims 21 to 23, characterized in that the NO₂ sorption is improved by using one or more suitable metal oxide additives. 25. A process according to any one of claims 21 to 24, characterized in that the molar ratio of SO₂ to NOX in the resulting gas stream above the absorption zone is greater than about 3.